The present invention relates to brake control systems for motor vehicles and more particularly, to a fail-safe arrangement in such a brake control system.
A conventional vehicle brake control system typically includes a depression (or brake operation) sensor for detecting the amount of operation of a brake foot pedal, electrical fluid pressure control valves for adjusting the pressure of a brake fluid to be transmitted from a source of fluid pressure to wheel cylinders, brake fluid pressure control means for controlling the operation of the electrical fluid pressure control valves based on output signals from the depression sensor, a master cylinder for transmitting to the wheel cylinders fluid pressure as developed in response to action of the brake foot pedal, electrical switch valves connected between the electrical fluid pressure control valves and the wheel cylinders and between the master cylinder and the wheel cylinders so as to selectively connect the wheel cylinders to either the electrical fluid pressure control valves or the master cylinder, and switch control means for controlling the operation of the electrical switch valves.
In such a conventional brake control system, the electrical fluid pressure control valves are operatively associated with either the respective wheels or a plurality of groups each comprised of a plurality of wheels. Also, the electrical switch valves are operatively associated with either the respective electrical fluid pressure control valves or a plurality of groups each comprised of a plurality of electrical fluid pressure control valves. The electrical fluid pressure control valves and the electrical switch valves are all connected through a common power supply relay to a power source.
Also, only one depression sensor is provided to detect the amount of operation of the brake foot pedal.
If, for example, an electrical failure occurs in any one of the electrical fluid pressure control valves, brake fluid pressure is inadvertently developed in the absence of brake action. For fail-safe purposes, the power supply relay is connected between all of the electrical fluid pressure control valves and the switch valves and the power source and adapted to interrupt the supply of electrical current to all of the electrical fluid pressure control valves and the switch valves. The electrical switch valves then enable brake fluid pressure developed in the master cylinder in response to action of the brake foot pedal to be transmitted directly to the respective wheels.
Such an arrangement is, however, incapable of providing power assistance. As a result, the driver is required to depress the brake foot pedal with a force significantly greater than that normally required. There is thus a need to provide an improved brake control system which provides a sufficient fail-safe function. A brake booster is useful in the event of an electrical system failure, but is highly costly.
Another problem is that the prior art brake control system is provided with only one depression sensor. If the depression sensor malfunctions, the system is no longer capable of providing a sufficient fail-safe function.
Accordingly, it is an object of the present invention to provide a vehicle brake control system which provides a sufficient fail-safe function.